This is a proposal to obtain a dedicated 137cesium irradiator at Brown University. This irradiator would replace an unoperable 280 kVp x-ray machine. The cesium irradiator possesses characteristics (e.g., high dose rate, collimated beams, relevant energy spectrum, depth dose characteristics and flexibility of configuration) that make it the best choice to fulfill the research needs of scientist users that range from basic investigations in the physics of ionizing radiation (e.g., microdosimetry and modeling), to studies of radiation damage and repair in vitro, (e.g, survival and repair of cells on both the molecular and cellular levels, assessment of chromosomal damage and sister chromatid exchanges, combined modality studies involving radiation, drugs, and hyperthermia, etc.), to studies of damage to organized tissues (e.g. animal lethality normal tissue damage and repair, solid tumor radiobiology, immunosuppression, etc.). Also, this cesium irradiator fulls a large need in an active teaching program supported by individual research grants, in the training of both undergraduate and predoctoral students in radiation biophysics. There is no irradiation facility available to use for our purposes, either at Brown University or its affiliated hospital, or in the State of RHode Island.